Conventional radar display systems utilize a cathode ray tube display which manifests on the display screen weather or target information or other significant events and spaced apart concentric circular or arcuate lines termed range marks, at successive radial distances from an origin. The range marks provide an indication to an operator of the distance from the radar antenna, which corresponds to the origin on the display screen, to targets or weather patterns of interest. In conventional radar systems the display pattern is created by successive sweep lines radiating from the origin. In such systems range marks are easily created by providing a momentary video signal representing each range mark at appropriate points along each scan line, the points being the same radial distance along each scan line.
Most modern radars, at least of the airborne weather displaying type, digitize information to be displayed and store in a digital memory the digitized information functionally for each scan line as a series of digital numbers each corresponding to desired luminance at successive points along the scan line. If the information is displayed as a series of radial sweep lines, as above described, range mark creating signals may be stored in the digital memory.
If the information is displayed in the form of a series of parallel sweep lines the range mark to range mark spacing varies from line to line as does the spacing from the origin of each sweep line to the portion of the first range mark in that line. Therefore unlike the relatively simple apparatus for generating range marks in radars which utilize radial sweep lines, apparatus for generating range marks in parallel scan radars is more complex. Parallel sweep radar indicators receive information at successive radial lines as do the more conventional radars. A radial to parallel sweep line converter is provided which converts each digital signal at a particular distance along a particular azimuth to a corresponding signal at a particular location along a particular parallel scan line, which signal is then stored in memory and caused to periodically refresh the display CRT. The same converter can be utilized to determine the location of each range mark along each sweep line, which information is then stored in digital memory along with the other information to be displayed.
Due to the cost of digital memories it is desired to keep the number of bits of storage at a minimum which results undesirably in relatively poor resolution on the display screen as compared to the resolution capability of a typical cathode ray tube display. It has been found empirically that such resolution for the display of weather information is adequate but such resolution results undesirably in only a course approximation to a circle for the range mark information which has been found unacceptable.